At the present time there is considerable medical interest in reviving patients from coma. The term "coma" is used to describe a human patient's state of profound insensibility, i.e., the patient is unconscious and immobile. A "deep coma" is a coma lasting over one week.
Coma is distinguished from other mental impairments such as "dementia", which is a mental decrease in functioning, including lessened memory, in which the patient is conscious and generally mobile.
Coma as a medical symptom may be the result of many causes, including drug reactions and cardiovascular stroke. However, it is believed that the most common cause of coma is head injury, for example, head trauma in automobile accidents. A patient in coma may be emotionally and physically devastating to the patient's family. In addition, the care of coma patients is costly, for example, $300 to $1000 a day for hospital care, and imposes a burden on the health system. There are many cases of traumatic brain injury each year, with many leading to coma, including many who remain in a persistent vegetative state that may last from several days to more than a year.
Historically, recovery from coma has been demonstrated in laboratory animals. Adametz showed that when the recticular formation was removed in steps, giving the brain a chance to reorganize itself rather than all at one time, animals would not lose consciousness. Recent research has further explored the plasticity of the brain in humans, such as the work of Tsubowuawa and has shown successful treatment of the coma state by direct brain stimulation (electrodes implanted within the brain).
In U.S. Pat. No. 4,702,254 to Zabara an electric patch is applied, in a surgical operation, to the vagus nerve (tenth cranial nerve). The patient's brain waves may be sampled, by scalp EEG electrodes, and when the brain waves are abnormal they indicate the imminence of a convulsion (epileptic seizure). At that time a voltage pulse train is applied to the vagus electrode patch to prevent the convulsion.
In U.S. Pat. No. 5,269,303 to Wernicke an electrical stimulation signal is applied to an implanted electrode on the vagus nerve to treat dementia. The Wernicke patent mentions that the brain's thalamic and cortical areas are reached by vagal stimulation and states "vagal stimulation can be beneficial in treating dementia in its effect on the recticular formation or activating system, the network of neurons involved in controlling the level of alertness" (column 2, lines 35-38).